1. Field of the Invention
The invention disclosed herein relates to exploration for oil and gas and, in particular, to gamma detectors adapted for a downhole instrument.
2. Description of the Related Art
In the exploration for oil and gas, it is necessary to drill a wellbore into the Earth. While drilling of the wellbore permits individuals and companies to evaluate sub-surface materials and to extract desired hydrocarbons, many problems are encountered.
Consider instrumentation that makes use of radiation detectors. The radiation detectors are typically included in such instruments to evaluate composition of the surrounding sub-surface materials and geologic formations. In some embodiments, radiation sources may be included in the instrument to provide a radiation signal. In other embodiments, the radiation detectors simply monitor background or natural radiation. One exemplary type of radiation detector is a gamma scintillator. Typically, a gamma scintillator is cylindrical and matched to a cylindrical photomultiplier (PMT) tube.
Typical radiation detectors used today, such as the gamma scintillator described above, do not have directional sensitivity. Accordingly, it is difficult for operators to determine directions of interest when a tool is down the wellbore (also referred to as “downhole”). As an accommodation for this, designers have incorporated shielding, which provides a degree of the desired directional sensitivity. Unfortunately, and particularly in the case of instruments using high energy radiation sources, the shielding required can be voluminous and therefore does not make an economic use of space within the instrument.
What are needed are methods and apparatus suited for providing downhole radiation detectors with directional sensitivity. Preferably, the methods and apparatus should provide for reductions or elimination of shielding systems.